Tunable capacitor

ABSTRACT

Disclosed are embodiments of a transistor that operates as a capacitor and an associated method of tuning capacitance within such a capacitor. The embodiments of the capacitor comprise a field effect transistor with front and back gates above and below a semiconductor layer, respectively. The capacitance value exhibited by the capacitor can be selectively varied between two different values by changing the voltage condition in a source/drain region of the transistor, e.g., using a switch or resistor between the source/drain region and a voltage supply. Alternatively, the capacitance value exhibited by the capacitor can be selectively varied between multiple different values by changing voltage conditions in one or more of multiple channel regions that are flanked by multiple source/drain regions within the transistor. The capacitor will exhibit different capacitance values depending upon the conductivity in each of the channel regions.

BACKGROUND

1. Field of the Invention

The embodiments of the invention generally relate to capacitors and, more particularly, to a capacitor structure that allows for modulation of the capacitance value.

2. Description of the Related Art

Various integrated circuit applications require the incorporation of tunable capacitors (e.g., in delay lines, load tuners, etc.). However, currently available tunable capacitor structures are limited in terms of continued size scaling (e.g., due to dopant fluctuations, current leakages, etc.). Therefore, there is a need in the art for an improved tunable capacitor structure that lends itself to continued size scaling comparable to that of other semiconductor devices (e.g., current state of the art field effect transistors (FETs)).

SUMMARY

In view of the foregoing, disclosed herein are embodiments of a back gate transistor that operates as a capacitor and an associated method of tuning capacitance within such a capacitor. The embodiments of the capacitor structure comprise a field effect transistor with front and back gates above and below a semiconductor layer, respectively. The capacitance value exhibited by the capacitor can be selectively varied between different values by changing the voltage condition in a selected region or regions of the capacitor. Specifically, the capacitor can comprise a semiconductor layer and a first gate on a first surface of the semiconductor layer and at least one second gate on a second surface of the semiconductor layer opposite the first gate. The semiconductor layer is adapted to change a capacitance value of the capacitor depending upon an amount of voltage that is either applied to a doped region adjacent to a channel region within the semiconductor layer or applied to one or more of multiple channel regions between multiple doped regions within the semiconductor layer.

That is, the capacitance value can be varied between two different values by changing the voltage condition within a doped source/drain region of the transistor. Applying a first voltage value (e.g., a value above a predetermined threshold) to the doped region causes the capacitor to exhibit a first capacitance value and applying a second voltage value (e.g., a value below the predetermined threshold) to the doped region causes the capacitor to exhibit a second capacitance value. Alternatively, the capacitance value exhibited by the capacitor can be selectively varied between multiple different values by changing voltage conditions on multiple channel regions that are flanked by multiple doped source/drain regions within the semiconductor layer (e.g., using a separate back gate to selectively control the threshold voltage of each channel region). The capacitor will exhibit different capacitance values depending upon the conductivity in each of the channel regions.

More specifically, in one embodiment of the structure, the capacitor comprises a field effect transistor. The transistor comprises a semiconductor layer having a first surface and a second surface. The semiconductor layer comprises a central channel region disposed by one, two, or more, doped source/drain regions. The transistor further comprises front and back gates above and below the channel region of the semiconductor layer (i.e., a first gate on a first surface of the semiconductor layer adjacent to the channel region and a second gate on a second surface of the semiconductor layer adjacent to the channel region and opposite the first gate). Each of the gates comprises a gate dielectric layer adjacent to the semiconductor layer and a gate conductor layer on the gate dielectric layer.

The capacitor further comprises a device (e.g., a switch, such as a pass-gate or transmission-gate, a resistor, an inductor, a digital-to-analog converter, etc,) that is adapted to selectively alternate (e.g., between a first voltage value and a second voltage value) an amount of voltage applied to one of the doped regions. Applying the first voltage value to the doped region causes the capacitor to exhibit a first capacitance value and applying the second voltage value to the doped region causes the capacitor to exhibit a second capacitance value.

In another embodiment of the structure, the capacitor similarly comprises a field effect transistor. The transistor comprises a semiconductor layer having a first surface and a second surface. The semiconductor layer comprises multiple channel regions and multiple doped source/drain regions. The channel regions and source/drain regions are configured such that each channel region is flanked by one, two, or more, doped regions. The doped regions can each be electrically connected to ground, to a supply voltage (Vdd), to an intermediate voltage, etc. The transistor further comprises front and back gates above and below the semiconductor layer (i.e., first and second gates, respectively). Each of the gates comprises a dielectric layer adjacent to the semiconductor layer and a conductive layer on the dielectric layer.

Specifically, the transistor can comprise a first gate that extends across the length of the semiconductor layer on the first side between the end-most doped regions (i.e., between a first doped region and last doped region). The first gate can be adapted to turn on the channel regions. Alternatively, the transistor can comprise a plurality of first gates adjacent to the first surface of the semiconductor layer. These multiple first gates are electrically isolated from each other and each one is positioned adjacent to a corresponding one of the channel regions between doped regions. Each first gate is adapted to turn on its corresponding channel region. Additionally, the transistor can comprise a plurality of second gates adjacent to the second surface of the semiconductor layer. These second gates are electrically isolated from each other and each one is positioned adjacent to a corresponding one of the channel regions between doped regions. Each one of the second gates is adapted to selectively control the threshold voltage of its corresponding channel region. Thus, any one or more of the channel regions within the transistor can selectively be turned on so that conductivity is high. Varying which, if any, of the channel regions is turned on will vary the capacitance value of the capacitor.

An embodiment of a method of selectively varying a capacitance value of a capacitor comprises providing a capacitor that comprises a field effect transistor with front and back gates above and below a semiconductor layer, respectively (i.e., at least one first gate on a first surface of the semiconductor layer and at least one second gate on a second surface of the semiconductor layer). Then, in order to vary the capacitance value of the capacitor, a voltage condition is changed in a selected region or regions of the transistor (e.g., in either a doped source/drain region within the semiconductor layer or on one or more multiple channel regions within the semiconductor layer. The voltage condition on a doped source/drain region can be changed by selectively alternating an amount of voltage applied to the doped region between a first voltage value and a second voltage value such that when the first voltage value is applied to the doped region the capacitor exhibits a first capacitance value and when the second voltage value is applied to the doped region the capacitor exhibits a second capacitance value. Alternatively, the voltage condition on one or multiple channel regions can be changed by using multiple back gates to selectively control the individual threshold voltages of the channel regions such that any one or more of the channel regions within the transistor can selectively be turned on so that conductivity is high. Varying which, if any, of the channel regions is turned on will vary the capacitance value of the capacitor.

These and other aspects of the embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments of the invention and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments of the invention without departing from the spirit thereof, and the embodiments of the invention include all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be better understood from the following detailed description with reference to the drawings, in which:

FIGS. 1 a-b are schematic diagrams illustrating an embodiment of the capacitor of the invention;

FIG. 2 is a schematic diagram illustrating another embodiment of the capacitor of the invention;

FIG. 3 is a schematic diagram illustrating an alternative structure for the embodiment of FIG. 2; and

FIG. 4 is a flow diagram illustrating embodiments of a method of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The embodiments of the invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments of the invention. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments of the invention may be practiced and to further enable those of skill in the art to practice the embodiments of the invention. Accordingly, the examples should not be construed as limiting the scope of the embodiments of the invention.

As mentioned above, various integrated circuit applications require the incorporation of tunable capacitors (e.g., in delay lines, load tuners, etc.). However, currently available tunable capacitor structures are limited in terms of continued size scaling (e.g., due to dopant fluctuations, current leakages, etc.). In complementary metal oxide semiconductor (CMOS) devices and, specifically, in field effect transistors, continued size scaling at or beyond the 32 nm node has recently been enabled by dual gate structures in which a back gate is used to adjust threshold voltage. This back-gate technology provides new opportunities for innovative structures and circuits.

In view of the foregoing, disclosed herein is a new solution for tuning capacitance in circuits by taking advantage of such back gate structures. Specifically, disclosed herein are embodiments of a back gate transistor that operates as a capacitor and an associated method of tuning capacitance within such a capacitor.

The embodiments of the capacitor structure (see capacitor 100 of FIGS. 1 a-b, capacitor 200 a of FIG. 2 and capacitor 200 b of FIG. 3) each comprise a field effect transistor with front and back gates above and below a semiconductor layer, respectively (i.e., at least one first gate 110, 210 on a first surface 153, 253 of the semiconductor layer 150, 250 and at least one second gate 120, 220 on a second surface 154, 254 of the semiconductor layer 150, 250). The capacitance value exhibited by the capacitor 100, 200 a, 200 b can be selectively varied by selectively changing the voltage conditions in different regions of the capacitor. Specifically, the capacitor can comprise a semiconductor layer and a first gate on a first surface of the semiconductor layer and at least one second gate on a second surface of the semiconductor layer opposite the first gate. The semiconductor layer is adapted to change a capacitance value of the capacitor depending upon an amount of voltage that is either applied to a doped region adjacent to a channel region within the semiconductor layer or applied to one or more of multiple channel regions between multiple doped regions within the semiconductor layer. For example, the capacitance value in capacitor 100 of FIGS. 1 a-b can be changed between two different values by changing the voltage condition in a doped region 151 within the semiconductor layer 150 (e.g., within one of the source/drain regions of the transistor). To accomplish this, the capacitor 100 may comprise a device 160 that is electrically connected between one of the doped source/drain regions 151 and a voltage supply 170. This device 160 can be adapted to selectively alternate an amount of voltage applied to the doped region 151 between a first voltage value and a second voltage value. Applying a first voltage value (e.g., a value above a predetermined threshold value) to the doped region 151 causes the capacitor 100 to exhibit a first capacitance value and applying a second voltage value (e.g., a value below the predetermined threshold) to the doped region 151 causes the capacitor 100 to exhibit a second capacitance value. Alternatively, the capacitance value exhibited by the capacitor 200 a of FIGS. 2 and 200 b of FIG. 3 can be selectively varied between multiple different values by changing voltage conditions on multiple channel regions 252 a-c that are flanked by doped regions 251 a-d (i.e., multiple source/drain regions) within the semiconductor layer 250. For example, the capacitor 200 a, 200 b can comprise multiple back gates 220 a-c, each of which is positioned adjacent to a corresponding one of the multiple channel regions 252 a-c within the semiconductor layer 250 and each of which is further selectively controlled to vary the threshold voltage of that corresponding channel region. The capacitor 200 a, 200 b will exhibit different capacitance values depending upon the conductivity in each of the channel regions.

More specifically, referring to FIGS. 1 a and 1 b, in one embodiment of the structure, the capacitor 100 comprises a field effect transistor (FET). The transistor comprises a semiconductor layer 150 having a first surface 153 and a second surface 154. The semiconductor layer 150 comprises a central region 152 (i.e., a channel region) flanked by two doped end regions 151 (i.e., disposed between two source/drain regions that are “floated” from an alternating current (AC) point of view). For example, if the transistor comprises an n-type FET, the doped regions 151 can be doped with an n-type dopant (e.g., phosphorus (P), arsenic (As) or antimony (Sb)). Alternatively, if the transistor comprises a p-type FET, the doped regions 151 can be doped with a p-type dopant (e.g., boron). The transistor further comprises front and back gates above and below the channel region of the semiconductor layer (i.e., a first gate 110 on a first surface 153 of the semiconductor layer 150 adjacent to the channel region 152 and between the doped regions 151 and a second gate 120 on a second surface 154 of the semiconductor layer 150 adjacent to the channel region 152 and between the doped regions 151, opposite the first gate 110). Each of the gates 110, 120 comprises a gate dielectric layer 112, 122 adjacent to the semiconductor layer 150 and a gate conductor layer 111, 121 on the dielectric layer. Thus, the capacitor 100 is formed between the front and back gates 110, 120.

The capacitor 100 further comprises a device 160 that is adapted to selectively alternate an amount of voltage (e.g., between a first voltage value and a second voltage value) that is applied from a constant (i.e., stable) voltage source to the one doped region. Applying the first voltage value to the doped region 151 causes the capacitor 100 to exhibit a first capacitance value and applying the second voltage value to the doped region 151 causes the capacitor 100 to exhibit a second different capacitance value.

For example, applying a first voltage that has a value above a predetermined threshold value to the doped region 151 (i.e., adjusting the potential of the doped region above the threshold voltage (Vt) for both the front and back gates 110, 120) causes the doped region 151 to remain fully depleted such that it functions as an insulator and no channel is formed. Furthermore, the potential of the doped region is limited so as not to become so positive that accumulation of the channel region by either or both gates occurs, by keeping the channel voltage below the flat-band voltage, with respect to the gates. Therefore, the series capacitance of the front dielectric 112, the back dielectric 122 and the body thickness 150, form a low-value capacitance between the gate conductors 111, 112. Specifically, referring to FIG. 1 a, the semiconductor layer 150 and gate dielectric layers 112 and 122 function as a composite dielectric between two conductive plates (i.e., gate conductors 111, 121 such that the capacitor 100 exhibits a low capacitance value (i.e., operates in a low capacitance (C) mode). More specifically, if Vg1−Vn+<Vt1 AND Vg2−Vn+<Vt2, then C=Cox1*Cox2*Cdep/(Cox1*Cox2+Cox2*Cdep+Cdep*Cox1), where Vg1 is the voltage condition at gate 110, Vn+ is the voltage condition at the doped region 151, Vt1 is the threshold voltage of gate 110 for the applied conditions, Vt2 is the threshold voltage of gate 120 for the applied conditions, Cox1 is the value of the capacitance from the gate 110 to the channel formed adjacent to gate 110, when inverted, Cox2 is the value of the capacitance from gate 120 to the channel formed adjacent to gate 120 when inverted, and Cdep is the value of the capacitance between the channels formed adjacent to gates 110 and 120. Furthermore, to avoid accumulation of the channel region, Vg1−Vn+>Vfb1 AND Vg2−Vn+>Vfb2, where Vfb1 and Vfb2 are the flat-band voltages of gate 110 and gate 120, respectively, to the channel region.

Whereas, applying a second voltage, having a value that is below the predetermined threshold value, to the doped region 151 (i.e., floating the potential of the doped region below the threshold voltage (Vt) for both the front and back gates 110, 120) changes the conductivity of the doped region to high and, thereby, allows inverted regions (i.e., channels that are formed) on the top surface 153 and bottom surface 154 of the semiconductor layer 150 to be electrically connected. Thus, referring to FIG. 1 b, instead of a composite dielectric that includes the semiconductor layer 150, capacitance is provided by the series combination of two dielectrics (i.e., the gate dielectrics 112, 122) between three conductive plates (i.e., the gate conductors 111, 121 and the semiconductor layer 150) such that the capacitor 100 exhibits a high capacitance value (i.e., operates in a high C mode). More specifically, if Vg1−Vn+>Vt1 AND Vg2−Vn+>Vt2, then C=Cox1*Cox2/(Cox1+Cox2), where the variables are the same as set out above.

In order to alternate the voltage values applied to the doped region 151, the device 160 can comprise an intermittent connection, such as a switch (e.g., a pass-gate switch, a transmission-gate, etc.), thereby allowing the voltage value to alternate on demand between a first voltage value that is above a predetermined threshold value and a second voltage value that is below a predetermined threshold voltage. Alternatively, the device 160 can comprise device, such as a resistor, an inductor with impedance that is large compared to that of the capacitor, or a digital-to-analog converter, that similarly allows the voltage applied to the doped region to alternate (i.e. be switched) between a value above a predetermined threshold value (i.e., the first voltage value) and a value below a predetermined threshold voltage (i.e., the second voltage value).

Referring to FIGS. 2 and 3, in another embodiment of the structure, the capacitor 200 a of FIGS. 2 and 200 b of FIG. 3 similarly comprises a field effect transistor. The transistor comprises a semiconductor layer 250 having a first surface 253 and a second surface 254. The semiconductor layer 150 comprises multiple channel regions 252 a-c and multiple doped regions 251 a-d (i.e., source/drain regions). For example, if the transistor comprises an n-type FET, the doped regions 251 a-d can be doped with an n-type dopant (e.g., phosphorus (P), arsenic (As) or antimony (Sb)). Alternatively, if the transistor comprises a p-type FET, the doped regions 251 a-d can be doped with a p-type dopant (e.g., boron). The channel regions 252 a-c and doped regions 251 a-d are configured such that they are alternate. Specifically, each channel region 252 a-c is flanked by two doped regions 251 a-d. The doped regions 251 a-d are each electrically connected to ground 290 or, alternatively, to a supply voltage (Vdd) or to an intermediate voltage. The transistor further comprises front and back gates above and below the semiconductor layer 250 (i.e., first and second gates 210, 220, respectively). Each of the gates 210, 220 comprises a dielectric layer 212, 222 adjacent to the semiconductor layer 250 and a conductive layer 211, 221 on the dielectric layer.

Specifically, referring to the capacitor 200 a of FIG. 2, the transistor can comprise a single first gate 210 that extends across the length of the semiconductor layer 250 on the first side 253 between the end-most doped regions (i.e., between a first doped region 251 a and last doped region 251 d). The single first gate 210 can be adapted to turn on all of the channel regions 252 a-c. Alternatively, referring to the capacitor 200 b of FIG. 3, the transistor can comprise a plurality of first gates 210 a-c adjacent to the first surface 253 of the semiconductor layer 250. These first gates 210 a-c are electrically isolated from each other and each one is positioned adjacent to a corresponding one of the channel regions 252 a-c between two doped regions 251. Each first gate 210 a-c is adapted to individually turn on its corresponding channel region 252 a-c.

Additionally, the transistor can comprise a plurality of second gates 220 a-c adjacent to the second surface 254 of the semiconductor layer 250. These second gates 220 a-c are electrically isolated from each other and each one is positioned adjacent to a corresponding one of the channel regions 252 a-c between two doped regions 251. Each one of the second gates 220 a-c is adapted to selectively control the threshold voltage (Vt) of the corresponding one of the channel regions 252 a-c. Thus, any one or more of the channel regions 252 a-c within the transistor can selectively be turned on so that conductivity is high within that given region. Varying which, if any, of the channel regions 252 a-c is turned on will vary the capacitance value of the capacitor (i.e., the capacitor will exhibit different capacitance values depending on the conductivity in each of the channel regions). That is, the capacitance between the top and bottom gates is a function of the aggregate number of channels established in the various channel regions between the doped regions. For example, if all of the back gates are off (i.e., each channel region has a high threshold voltage), the capacitance value will be low because the only capacitance exhibited is between the gates and individual doped regions. However, by turning on a single back gate (e.g., 220 a), a parallel plate dielectric is added in the corresponding section of the capacitor and, thereby, adds capacitance. Consequently, with each back gate 220 a-c that is turned on simultaneously, additional capacitance is added.

In this mode of invention, the electrical biases are provided to each back gate 220 a-c via an effectively high impedance element such as a pass-gate, a transmission-gate, a high-valued resistor, an inductor or an analog-to-digital converter, as discussed in the earlier modes.

Referring to FIG. 4, an embodiment of a method of selectively varying a capacitance value of a capacitor comprises providing a capacitor, e.g., as illustrated in FIG. 1, 2 or 3 (402). Specifically, a capacitor that comprises a field effect transistor with front and back gates above and below a semiconductor layer, respectively (i.e., at least one first gate on a first surface of the semiconductor layer and at least one second gate on a second surface of the semiconductor layer) is provided. Then, in order to vary the capacitance value of the capacitor, a voltage condition is changed on either a doped region within the semiconductor layer or on one or more of multiple channel regions within the semiconductor layer (404).

For example, a capacitor 100 with a channel region 152 between source/drain regions 151, a single front gate 110 on one side 153 of the channel region 152 and a single back gate 120 on the opposing side 154 of the channel region 152 can be provided (412, see FIGS. 1 a-b and detailed discussion above). The voltage condition on one of the source/drain regions 151 (i.e., a doped region) of the capacitor 100 can be changed by selectively alternating an amount of voltage applied to the doped region 151 between a first voltage value and a second voltage value such that when the first voltage value is applied to the doped region the capacitor exhibits a first capacitance value and when the second voltage value is applied to the doped region the capacitor exhibits a second different capacitance value (414). For example, for an n-type device, applying a first voltage that has a value greater than a predetermined threshold value to the doped region 151 (i.e., adjusting the potential of the doped region above the threshold voltage (Vt) for both the front and back gates 110, 120) causes the channel region 152 to remain fully depleted such that it functions as an insulator and no channel is formed. Furthermore, the potential of the doped region is limited so as not to become so positive that accumulation of the channel region by either or both gates occurs, by keeping the voltage of the doped region below the flat-band voltage with respect to the gates 110, 120. Therefore, the series capacitance of the front dielectric 112, the back dielectric 122 and the body thickness 150, form a low-value capacitance between the gate conductors 111, 112 (see FIG. 1 a and discussion above). Whereas, applying a second voltage, having a value less than the predetermined threshold value, with respect to the doped region 151 (i.e., floating the potential of the doped region below the threshold voltage (Vt) for both the front and back gates 110, 120) changes the conductivity of the channel region to a high value and, thereby, allows inverted regions (i.e., channels that are formed) on the top surface 153 and bottom surface 154 of the semiconductor layer 150 to be electrically connected via the doped region(s). Thus, referring to FIG. 1 b, instead of a composite dielectric that includes the semiconductor layer 150, capacitance is provided by the series combination of two dielectrics (i.e., the gate dielectrics 112, 122) between three conductive plates (i.e., the gate conductors 111, 121 and the semiconductor layer 150) such that the capacitor 100 exhibits a high capacitance value (i.e., operates in a high C mode). This process 414 can be accomplished by either intermittently connecting the doped region to a constant (i.e., stable) voltage source, e.g., using a switch, such as a transmission-gate or pass-gate, (415) or via a sufficiently high value of resistance or inductance between the voltage source and doped region (e.g., using a resistor, inductor, analog-to-digital converter, etc.) (416). In particular, a resistor can be used if the resistor has a value R>>t/C, where t is the effective time scale which the capacitor must function in the circuit, and similarly the inductance can be of value L>>t²/C, where C is the value of capacitance in use.

Alternatively, a capacitor 200 a or 200 b with multiple channel regions 252 a-c disposed between multiple source drain regions 251 a-d within a semiconductor layer 250, a single or multiple first gates 210, and multiple second gates 220 a-c can be provided (422, see FIGS. 2 and 3 and detail discussion above). The multiple second gates can be positioned on the second surface 254 of the semiconductor layer 250 such that each one of the second gates 220 a-c corresponds to one of the channel regions 252 a-c. Then, the voltage condition on any one or more of the multiple channel regions can be changed by selectively controlling the threshold voltage of each of the multiple channel regions (424). Thus, any one or more of the channel regions within the transistor can selectively be turned on so that conductivity is high.

Varying which, if any, of the channel regions 252 a-c is turned on will vary the capacitance value of the capacitor (i.e., the capacitor will exhibit different capacitance values depending on the conductivity in each of the channel regions). That is, the capacitance between the top and bottom gates is a function of the aggregate number of channels established in the various channel regions between the doped regions. For example, if all of the back gates are off (i.e., each channel region has a high threshold voltage, as determined by a low voltage on the back gate of that region), the capacitance value will be low because the capacitance exhibited is between the gates via the channel region. However, by turning on a single back gate (e.g., 220 a), a parallel plate dielectric is added in the corresponding section of the capacitor and, thereby, adds capacitance. Consequently, with each back gate 220 a-c that is turned on simultaneously, additional capacitance is added. In this mode of invention, the electrical biases are provided to each back gate 220 a-c via an effectively high impedance element such as a pass-gate, a transmission-gate, a high-valued resistor, an inductor or an analog-to-digital converter, as discussed in the earlier modes.

Therefore, disclosed herein are embodiments of a back gate transistor that operates as a capacitor and an associated method of tuning capacitance within such a capacitor. The embodiments of the capacitor structure comprise a field effect transistor with front and back gates above and below a semiconductor layer, respectively. The capacitance value exhibited by the capacitor can be selectively varied between two different values by changing the voltage condition in a source/drain region of the transistor, e.g., using a switch, a resistor, an inductor, analog-to-digital converter, etc., between the source/drain region and a voltage supply. Alternatively, the capacitance value exhibited by the capacitor can be selectively varied between multiple different values by changing voltage conditions one or more of multiple channel regions that are flanked by multiple source/drain regions within the transistor. The capacitor will exhibit different capacitance values depending upon the conductivity in each of the channel regions. The tunable capacitors disclosed herein can be incorporated into integrated circuits structures as delay lines, load tuners, etc. However, given that the structure of these tunable capacitors is based on that of current state of the art back gate transistor technology, size scaling of the capacitors can be in line with that achievable with back gate transistors such that they are not subject to the same limitations imposed by dopant fluctuations, leakage limits, etc., seen with prior art tunable capacitors.

Benefits that flow from this invention include improved flexibility in capacitive loading and coupling of circuits, improved isolation of control elements from active elements which results in improved quality factor.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments of the invention have been described in terms of embodiments, those skilled in the art will recognize that the embodiments of the invention can be practiced with modification within the spirit and scope of the appended claims. 

1. A capacitor comprising: a semiconductor layer having a first surface and a second surface, wherein said first surface is opposite said second surface, and wherein said semiconductor layer comprises one of a single channel region adjacent to a doped region and multiple channel regions between multiple doped regions; a first gate on said first surface; and at least one second gate on said second surface, wherein said semiconductor layer is adapted to change a capacitance value of said capacitor depending upon an amount of voltage applied to one of said doped region and at least one of said multiple channel regions.
 2. The capacitor of claim 1, further comprising a device adapted to selectively alternate said amount of voltage applied to said doped region between a first voltage value and a second voltage value.
 3. The capacitor of claim 2, wherein, when said first voltage value is applied to said doped region, said capacitor exhibits a first capacitance value and wherein, when said second voltage value is applied to said doped region, said capacitor exhibits a second capacitance value that is different from said first capacitance value.
 4. The capacitor of claim 1, further comprising multiple second gates each positioned adjacent to a corresponding one of said multiple channel regions and wherein each one of said second gates is adapted to selectively control a threshold voltage of said corresponding one of said channel regions.
 5. The capacitor of claim 1, wherein said capacitor exhibits different capacitance values depending upon the conductivity in each of said channel regions.
 6. A capacitor comprising: a semiconductor layer having a first surface, a second surface opposite said first surface, a center region extending from said first surface and said second surface and at least one doped region adjacent to said center region; opposing gates on said first surface and said second surface adjacent to said center region; and a device that is adapted to selectively alternate an amount of voltage applied to said doped region between a first voltage value and a second voltage value.
 7. The capacitor of claim 6, wherein, when said first voltage value is applied to said doped region, said capacitor exhibits a first capacitance value and wherein, when said second voltage value is applied to said doped region, said capacitor exhibits a second capacitance value that is different from said first capacitance value.
 8. The capacitor of claim 6, wherein each of said gates comprise a gate dielectric layer adjacent to said semiconductor layer and a gate conductor layer on said gate dielectric layer.
 9. The capacitor of claim 6, wherein said device comprises one of a switch, a resistor and an inductor.
 10. The capacitor of claim 9, wherein said switch comprises one of a pass-gate switch and a transmission-gate switch.
 11. The capacitor of claim 6, wherein said first voltage value is above a predetermined threshold voltage to form a relatively low-value capacitance and said second voltage value is below said predetermined threshold voltage to form a high-value capacitance relative to said low-value capacitance.
 12. A capacitor comprising: a semiconductor layer having a first surface and a second surface opposite said first surface and comprises at least three doped regions and channel regions between said doped regions; a first gate on said first surface; and a plurality of second gates on said second surface, wherein said second gates are electrically isolated from each other and wherein each one of said second gates is positioned adjacent to a corresponding one of said channel regions between two of said doped regions.
 13. The capacitor of claim 12, wherein each one of said second gates is adapted to selectively control a threshold voltage of said corresponding one of said channel regions.
 14. The capacitor of claim 12, wherein said capacitor exhibits different capacitance values depending on a conductivity in each of said channel regions.
 15. The capacitor of claim 12, further comprising a plurality of first gates on said first surface, wherein said first gates are electrically isolated from each other and wherein each of said first gates is positioned between a corresponding two of said doped regions opposite a corresponding one of said second gates.
 16. A method of selectively varying a capacitance value of a capacitor, said method comprising: providing a capacitor comprising: a semiconductor layer having a first surface and a second surface; a first gate on said first surface; and at least one second gate on said second surface; and one of changing a voltage condition on a doped region within said semiconductor layer and changing a voltage condition on at least one of multiple channel regions within said semiconductor layer.
 17. The method of claim 16, wherein said changing of said voltage condition on said doped region comprises selectively alternating an amount of voltage applied to said doped region between a first voltage value and a second voltage value.
 18. The method of claim 17, wherein, when said first voltage value is applied to said doped region, said capacitor exhibits a first capacitance value and wherein, when said second voltage value is applied to said doped region, said capacitor exhibits a second capacitance value.
 19. The method of claim 16, wherein said providing of said capacitor further comprises providing a capacitor with multiple second gates that are positioned on said second surface such that each one of said second gates corresponds to one of said channel regions and wherein said changing of said voltage condition on said at least one of said multiple channel regions comprises selectively controlling threshold voltages of said multiple second gates.
 20. The method of claim 19, wherein said capacitor exhibits different capacitance values depending upon the conductivity in each of said channel regions. 